A Tale of 7 Elements: Element 85—Astatine [Excerpt]

Excerpted from A Tale of 7 Elements, by Eric Scerri. Reprinted with permission of Oxford University Press, June 2013.

The story surrounding element 85 is one of the most complex and interesting among our seven elements (fig. 8.1). The various claims for its discovery reveal many of the nationalistic traits that we have seen in the case of other elements, most notably the controversy surrounding the discovery of hafnium, element 72.

But element 85 gives our study a greater depth than has yet been revealed by the already covered elements. What this story shows is that the nationalistic prejudices persist to this day in many respects and that the identity of the “discoverer” of the element very much depends on the nationality of the textbook that one might consult. It is also an element for which the majority of sources give an incorrect account in declaring Corson, MacKenzie, and Segrè as the true discoverers. The account I will detail owes much to the recent work of two young chemists, Brett Thornton and Shawn Burdette, whose 2010 article I have drawn heavily from.

As in the case of many of the seven elements already surveyed, the view that Moseley’s experimental demonstration of the concept of atomic number resolved all issues in a categorical fashion is once again shown to be highly misleading.

Early Claims for Element 85
The position of element 85 in the periodic table shows it to lie among the halogens. Not surprisingly, therefore, the early researchers believed that they would find the element in similar locations to other halogens such as bromine and iodine, namely in the oceans or in sands washed up by oceans. Moreover, it was fully expected that the new element would behave like a typical halogen to form diatomic molecules and that it would have a low boiling point.

The first major claim for the discovery of the element was made by Fred Allison, the same researcher who also erroneously claimed that he had discovered element 87. And just as in the case of element 87, Allison claimed to have found the new element using his own magneto-optical method, involving a time delay in the Faraday effect, which is to say the rotation of plane polarized light carried out by the application of a magnetic field to any particular solution of a substance. Allison published articles in 1931 and 1932 claiming that he had observed element 85 and proposing to call it alabamium after Alabama, the state in which he worked.

In 1935, the American physicist H. G. MacPherson showed that Allison’s findings were spurious and due to imperfections in his instruments rather than to the presence of a new element. Further refutations followed in quick succession.

The next claim came from Rajendralal De, an Indian chemist working in Dacca, then part of British India and now in Bangladesh. De had trained in Germany with Hahn and Meitner in the 1920s and like Allison, used monazite sand for his research. After applying a number of chemical processes to the sand, De obtained a sublimate that he claimed to be element 85 and to which he gave the name of dakin after the city of Dacca, also spelled Dhaka. Later researchers dismissed De’s claim on the basis of the powerful radioactivity of astatine, which would have prevented him or anybody else from safely handling the element in the manner he claimed to have done at the time.

Another person who had been involved in the search for element 87, Romanian Horia Hulubei, was also involved in the discovery of element 85. Indeed it appears that he may well have been the discoverer of naturally occurring astatine, as it was later called by the physicists who synthesized the element artificially. It is these physicists who are generally accorded with the discovery of the element.

Hulubei studied in France starting in 1916, returning to his native Romania after World War I had ended in 1918. In 1926, he came back to France to work with Jean Perrin and built an X-ray laboratory at the Sorbonne University. In 1928, they were joined by Yvette Cauchois, who built what later became known as the Cauchois spectrometer, which provided higher resolution spectra and made possible the study of weaker spectra than had previously been observed. Hulubei and Cauchois examined the radioactivity of radon in the hope of observing evidence of the presence of element 85. In a paper published in 1936, they claimed to have observed a line at 151 X-units, or siegbahns, precisely where the Kα1 line for eka-iodine was expected. In 1939, they reported two further X-ray lines consistent with the presence of eka-iodine and the predictions from Moseley’s law. These new experiments used higher resolutions than the earlier ones and included further checks and balances, which led to greater confidence in the authors’ claims to having discovered the new element. In 1941, a former student of Hulubei and Cauchois, Manuel Valadares, repeated the experiments with a stronger X-ray source after returning to his native Portugal. He then published his results, which also suggested the presence of eka-iodine.

In 1942, additional scientists entered the discussion on the new element. Two women, Berta Karlik and Traude Bernert, working at the Institute for Radium Research in Vienna, reported the detection of α particles emanating from the radioactive decay of a radon isotope. They also took this decay to indicate the presence of element 85 in part of a natural radioactive decay series. By this time the artificial synthesis of element 85, which is generally considered to be the definitive discovery of the element, had been conducted at Berkeley. The Austrian researchers were unaware of this fact, however, due to lack of communication during wartime.

In an article of 1944, Hulubei wrote a detailed summary of his work and that of others on element 85. This included a description of six X-ray lines that were thought to be due to natural radioactive decay producing the new element. He also appealed to the work of Karlik as providing support for his own findings. This time Hulubei went as far as to suggest a name for the new element, “dor,” which he took from the Romanian word for “longing” in the sense of “longing for peace.” This name represented an interesting shift away from naming elements in a nationalistic manner that had prevailed in the recent past.

As World War II drew to a close and some elements began to be produced artificially, it became important to decide on how elements should be named and who would have the right to give them new names. This task was taken up by the Austrian-born radiochemist Friedrich Paneth, who had fled from Berlin to the United Kingdom in 1936 after being dismissed from his professorship because of his Jewish origins. Paneth published an editorial in Nature magazine in 1947, which among other things would have the effect of depriving any discovery claims from Hulubei and Cauchois. As mentioned before, Paneth suggested that in cases in which an element had been given different names by competing groups, the naming rights should go to those who produced the element in a reproducible fashion. This meant that, in the case of element 43, the Noddacks’ claim for masurium should be dismissed and should be replaced by technetium, as synthesized by Segrè and Perrier.

Paneth noted the claim by the Berkeley group for the synthesis of element 85 and also the fact that Karlik and Bernert had showed that it exists in natural sources. But he went on to state that what he called “former claims,” without naming any particular researchers, had been disproved by the work of Karlik and Bernert. This is a rather crucial statement because it served to discredit the work of Hulubei and Cauchois, even though Karlik and Bernert had not actually addressed these claims whereas Paneth’s statement implied that they had.

Hulubei was understandably very concerned with Paneth’s editorial and the implication that his work and that of Cauchois had been refuted. He responded by attributing Paneth’s omission to the difficulties in communication during the war. He denied that Karlik and Bernert had refuted his research on element 85, adding the words, “contrary to what one would think after reading the expose of Mr. Paneth.” Soon afterward Karlik finally did comment on Hulubei’s work, claiming that the research had been insufficient to merit the discovery of element 85 because of the very small amount of element 85 in their sample, which would render likely some interferences from other elements in the X-ray spectra.

Meanwhile, in response to Paneth’s editorial, three Berkeley researchers claiming to have produced element 85 artificially— Corson, MacKenzie, and the previously mentioned Emilio Segrè— proposed the name “astatine” from the Greek astatos, or unstable. The authors had not been aware of the claims from Hulubei and Karlik but had delayed proposing a name for the element because of the continuing claims for alabamine by Allison and his supporters. Furthermore, Paneth, who was by now the chair of the committee of the International Union of Chemistry, approved the name of astatine in 1949, thus further lending his support to the American claim.

According to the analysis of Thornton and Burdette, there is no doubt that three teams of researchers can claim to have discovered element 85. First of all, they state that:

Unlike other flawed studies with X-ray spectroscopy, Hulubei and Cauchois indisputably had element 85 in their samples. The only uncertainty is whether their instrument was sensitive enough to distinguish the spectral lines of element 85.

One additional argument they offer for this claim is that, in the 1930s, Hulubei and Cauchois were able to clearly detect the Lα line for the element polonium, which has a 500-fold lower transition intensity than the lines they claimed to have seen in the case of element 85. Moreover, they add that the experiments carried out by their Portuguese student, Valadares, would have tripled the intensity in the claimed X-ray lines for element 85 because he used a radon source, which is three times more powerfully radioactive.

The reasons why Hulubei and Cauchois have never received much credit for their work have already been mentioned. They include Paneth’s disparaging words to the effect that “other work” on element 85 had been refuted even though Hulubei and Cauchois’s work had not. In addition, Thornton and Burdette attribute the lack of credit to the fact that Hulubei in particular had falsely claimed the discovery of element 87 and that he had definitely been wrong in that case. They propose that this earlier error caused others to doubt Hulubei, even though he had detected element 85.

Helvetium and Anglohelvetium
In 1940, the Swiss physicist Walter Minder (1905–1992), claimed to have observed an extremely weak β decay of radium A. For this purpose he connected a couple of ionization chambers with an electrometer. He also believed that his chemical tests confirmed the analogies of this element with iodine. Minder named it helvetium and gave it the symbol Hv, after the Latin name for Switzerland. Nature Magazine reported Minder’s findings in an abstract by announcing that he had succeeded in isolating element 85 and that he had done so from the decomposition of the radioactive element actinium. The abstract also noted that Minder had named his new element helvetium to honor his own country. It continued by expressing the hope that further details would soon be available, adding that the London Evening News had remarked,

It is odd to learn today, in the midst of war, that a patient Swiss chemist has succeeded at last in isolating the elusive chemical element ‘85.’ It is still odder that in the long view of history a discovery of that sort may rank above all the perils and victories of these days.

Then in 1942, Minder with his British colleague, Alice Leigh-Smith,12 surprisingly repeated the announcement of the discovery of eka-iodine, this time calling it anglohelvetium, a combination of Anglia (Latin for England) and Helvetia. But again others could not replicate these claims and not much was heard from these researchers again, at least in the context of the discovery of missing elements.

The Usually Acknowledged Discovery of Element 85
The discovery of element 85 was made by three Berkeley scientists, Dale Corson, Alexander MacKenzie, and Emilio Segrè, in 1940 (fig. 8.2). Using a 60-inch cyclotron built by Ernest Lawrence, the three scientists bombarded a target of bismuth, element 83.13 This element is rather useful in this context because of its having just one single isotope, with mass number 209, a feature that greatly simplifies the analysis of products. The bombarded bismuth target displayed a number of forms of radiation, including the emission of α, γ, X-rays and even low energy electrons, all exhibiting the same lifetime of about seven-and-a-half hours. Through a series of analyses the authors were able to identify the substance causing some of these radiations, with element 85 changing into polonium via K-electron capture.

Interestingly, in the article announcing their discovery, they also remarked about the possible existence of naturally occurring element 85 and cited the earlier work of Minder in Switzerland, as well as Hulubei and Cauchois in Paris, both of whom had claimed to have observed the element.

They also mentioned the work carried out with Hamilton and Soley in which element 85 was concentrated into the thyroid glands of some guinea pigs, showing similar excretion to that of iodine, which occurs above element 85 in the periodic table. Nevertheless, the chemical experiments of Corson et al. revealed that the proper- ties of element 85 are more similar to those of neighboring element 84 or polonium than they are to iodine. For example, element 85 precipitates as a sulfide and is precipitated by zinc in sulfuric acid, both of which are reactions that are characteristic of a metal rather than a nonmetal such as iodine.

General Aspects of Astatine
Element 85 has the dubious distinction of being one of very few solid elements that has never been obtained in any amount large enough to be visible to the naked eye. It is also estimated that if a visible sample were ever produced, it would immediately vaporize away due to the heat generated by the emitted radioactivity. As a result of these properties the bulk behavior of astatine, such as its melting and boiling points, its color, and the degree to which it may be a metal can only be estimated theoretically.

Based on melting point trends among the halogen elements, the value for astatine is predicted to be 302°C, although there is some controversy surrounding this work, as there is around the predicted melting point of 337°C. Another controversy concerns the apparently simple question of whether diatomic molecules of At2 occur as they do in the case of all the other halogens. The color is expected to be very dark and most probably black on the basis of the trend among the halogen group, to which astatine belongs. This is because fluorine is almost colorless to yellow, chlorine is green, bromine brown, and iodine a violet color.

In 1943, three years after astatine was first synthesized artificially in a nuclear reactor, it was discovered that the element occurs naturally in miniscule amounts in the earth’s crust. In fact it is the single rarest naturally occurring element, with a total of just 1 oz. or 28 grams at any given time. About thirty isotopes of the element have been synthesized or found to occur naturally, the longest-lived of which is 210At with a half-life of 8.1 hours.

Taken all together these facts about the element contribute to its almost complete lack of applications. One exception has been an ongoing exploration of the potential uses of 211At in radiotherapy. The isotope is an α particle emitter with a convenient half-life of 7.2 hours. Like the element above it in the periodic table, iodine, astatine has a tendency to be metabolized in the thyroid gland and could therefore be used to monitor medical conditions involving the thyroid and the throat area in general. In addition, the short-range nature of the α emission of this isotope suggests that it could be used to treat cancers in all parts of the body while reducing the risk to neighboring tissue that is often a problem in the use of other more established radio-therapeutic isotopes. And if that were not promise enough, 211At does not produce any harmful β radiation as do many other isotopes currently used in radio-medicine.

But although these therapeutic potentially attractive properties have been explored for more than thirty-five years, problems concerning the safe delivery of 211At to human subjects, as well as issues relating to the ready production of the isotope, continue to delay the in vivo implementation of this rarest of all elements.